Severity of heart attack is dependent on the time of day

The size of a heart attack and subsequent left-ventricular function are significantly different based on the time of day onset of ischemia, according to a first of its kind study in humans, published online Nov. 17 in Circulation Research. The greatest amount of injury to the heart occurs when individuals have a heart attack between 1:00am and 5:00am.

In previous research, the infarct size in rodents following ischemia and reperfusion exhibited a circadian dependence on the time of coronary occlusion. A circadian rhythm is an endogenously driven, roughly 24-hour cycle in biochemical, physiological or behavioral processes. Previously, it was unknown if a similar circadian dependence of infarct size occurs in humans.

"We were trying to ascertain whether the time of day of when a heart attack occurs influences the amount of damage that the heart sustains, or was this just a phenomenon exhibited in rodents," said the study's senior author Jay H. Traverse, MD, a cardiologist at the Minneapolis Heart Institute at Abbott Northwestern Hospital in Minneapolis and physician researcher with Minneapolis Heart Institute Foundation.

All 165 patients had well-defined ischemic times and the data were supported by a subgroup with cardiac MRI measurements of infarct size, or size of heart attack, and area-at-risk.

The researchers observed that the extent of infarct size was significantly associated with time of day onset of infarction. The greatest myocardial injury occurred at a 1:00am onset of ischemia and 5:00am onset of reperfusion with the peak injury being 82 percent higher than that recorded at lowest time of injury.

What are the implications of these findings? "It is important to understand that the heart's ability to protect itself against more severe damage varies over a 24-hour cycle. Identifying those protective changes may be particularly relevant for pharmaceutical manufacturers that are seeking to develop cardioprotective drugs," Traverse explained.

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